In the field of silver halide photographic emulsions, spectral sensitization is a technique for extending the sensitivity range of silver halide grains from their inherent spectral absorption range to long wavelength range such as the visible light range and the infrared range. Spectral sensitization is therefore essential for the preparation of silver halide photographic materials. On the other hand, high sensitivity and high quality silver halide photographic materials are still highly desired. Furthermore, with the recent remarkable progress in many fields such as electronics, as represented by the development of various light sources such as LED, laser and CRT, silver halide photographic materials suited for these light sources are now desired. Thus, silver halide photographic materials suited for various systems are desired.
To prepare spectrally sensitized silver halide emulsions, a sensitizing dye is normally incorporated during the time following the completion of the chemical ripening of the silver halide emulsions, but before the coating thereof on a proper support. However, some kinds of silver halide grains or sensitizing dyes often undergo a sensitivity change during their storage after the addition of a sensitizing dye, but before coating or during the storage after coating. It has been desired to overcome this difficulty, and many approaches have been proposed. For example, methods disclosed in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756 and 4,225,666 propose the addition of a sensitizing dye during the formation of silver halide grains before the completion of the grain formation process. Methods disclosed in U.S. Pat. No. 4,442,201 (corresponding to JP-A-58-7629 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application")) and JP-A-59-9658, JP-A-59-48756 and JP-A-59-113920 propose the addition of a sensitizing dye before or during chemical ripening of silver halide grains. It is said that the use of these approaches enables not only an increase in photographic sensitivity but also an improvement in the adsorption of a sensitizing dye by silver halide grains, eliminating the desensitization which otherwise occurs during the storage of a silver halide emulsion in the solution state before coating. Thus, these approaches are often used.
However, even these approaches leave much to be desired. In these approaches, photographic desensitization occurring during the storage of a spectrally sensitized emulsion in the solution state before coating or during the storage of the emulsion after coating cannot be often substantially eliminated. Furthermore, it is often observed that fog is more developed than is the case where a sensitizing dye is incorporated after the completion of chemical ripening.
In order to inhibit fogging during storage of a silver halide photographic material or during chemical ripening or to prevent an increase in fogging during development, most silver halide photographic materials normally contain a so-called fog inhibitor (called an "emulsion stabilizer" for the former purpose or a "development inhibitor" for the prevention of fog during development). Like the spectral sensitization technique, this fog inhibition process is essential for the preparation of silver halide photographic materials.
If an effective amount of such a fog inhibitor is used to inhibit fog caused by the above mentioned approaches, a remarkable sensitivity drop or a remarkable desensitization increase during the storage of the materials occurs. This eliminates the disclosed advantages. Furthermore, the results are often poorer than is the case where a sensitizing dye is incorporated after chemical ripening. Thus, it has been difficult to use these approaches.
The adsorption of a sensitizing dye by silver halide grains is often competitive with the adsorption of a fog inhibitor by silver halide grains. Therefore, the sensitizing dye may be desorbed by the fog inhibitor, or the fog inhibitor may be in turn desorbed by the sensitizing dye. The former phenomenon causes a spectral sensitivity drop, and the latter causes a fog increase. This competition means that the photographically most desirable sensitizing dye and fog inhibitor cannot be freely selected.
In general, the adsorption of an ordinary cyanine dye by silver halide grains is mainly based on van der Waals forces. It has been observed that as the polarizability of the base decreases, these forces decrease. In particular, the adsorption decreases in the order of AgI, AgBr and AgCl. On the other hand, most fog inhibitors are adsorbed by silver halide grains more strongly to the extent that the solubility product (Ksp) of their silver salts or silver complexes is smaller than the solubility product of silver halide. Therefore, it is known that if the same fog inhibitor is used, it can be adsorbed more strongly by AgBr than by AgI and more strongly by AgCl than by AgBr. Accordingly, if a silver halide emulsion comprising a high surface Cl content is used, a fog inhibitor tends to desorb a cyanine dye. Therefore, even if the above mentioned techniques for the addition of a cyanine dye are used, desorption of a cyanine dye can be easily caused, resulting in a sensitivity drop. Furthermore, a sensitizing dye belonging to a so-called merocyanine dye or a complex cyanine dye is similarly competitive with a fog inhibitor for adsorption by silver halide grains.
It has been proposed that the captured quantity of light be increased to improve the sensitivity in the wavelength range of absorption of a sensitizing dye by a silver halide emulsion. In this respect, it is advantageous to use tablet grains having a large specific surface area as disclosed in JP-A-58-127921 and JP-A-58-113927. However, even with the use of these silver halide grains, as the percentage of adsorption of a sensitizing dye increases, the inherent sensitivity of the emulsion decreases. Even if the captured quantity of light is increased accordingly, the efficiency of sensitization by the sensitizing dye decreases. The resulting photographic sensitivity is not necessarily high enough. One of the reasons for this trouble is probably that the aggregates formed by the excessive aggregation of sensitizing dyes can easily serve as electron trapping centers. The above mentioned approaches which comprise the addition of a sensitizing dye before the completion of chemical ripening often make it easier to form excessive aggregates of sensitizing dyes on the surface of silver halide grains than do the approaches which comprise the addition of a sensitizing dye after the completion of chemical ripening.
Most of the high aspect ratio tabular grains have a (111) plane as the main plane. However, most sensitizing dyes are adsorbed more weakly by a (111) plane than the (100) plane of AgBr, AgBrCl or AgCl grains. Accordingly, if such silver halide grains having a (111) plane are used, desorption can be caused not only by a fog inhibitor but also by a coupler or its emulsion dispersion or a surface active agent as coating aid to be used in color light-sensitive materials. This can cause a photographic sensitivity drop during the storage of a spectrally sensitized emulsion in the solution state before coating or during the storage of the emulsion after coating.
As a result of intensive studies to change the adsorption of a sensitizing dye and a fog inhibitor by silver halide grains from competitive to cooperative relationship, the inventors have discovered a process to overcome these problems. In particular, the inventors found a process for the preparation of a silver halide photographic emulsion which provides a high spectral sensitivity without generation of fog and enables the remarkable elimination of a sensitivity drop of a spectrally sensitized emulsion during the storage in the solution state before coating and during the storage thereof after coating.